Currently, the 1.5 PDF file is processed by displaying with Okular, printing it to PDF (which produces a version 1.4 pdf file) and then using pdftk on this "printed" file. It works but that's an ugly way of doing things.

The problem is that pdftk uses an internal itext library which is quite outdated. So the solution would be to make it use external dev-java/itext, or a workaround to specify the PDF version in your LaTeX files.